Gas sensor

ABSTRACT

A gas sensor for detecting at least one physical magnitude of a gas, in particular of exhaust gases of an internal combustion engine, is proposed, which includes a sensor element having an electrochemical cell. The electrochemical cell includes a first solid electrolyte member on which a first electrode and a second electrode are applied. The first and the second electrode are electrically connected by means of the first solid electrolyte member. The first electrode is in contact with the gas. The area of the first electrode is smaller than the area of the second electrode.

This application is a 371 National Stage Entry of PCT/DE02/03770 filedon Oct. 4, 2002.

FIELD OF THE INVENTION

The present invention is based on a gas sensor.

BACKGROUND INFORMATION

A gas sensor is known from the German Published Patent Application No.199 41 051, for example, for use in analyzing the exhaust gas ofinternal combustion engines. The gas sensor regulates the air/fuel ratioof combustion mixtures in motor vehicle engines and is provided with asensor element that combines a concentration cell (Nernst cell) with anelectrochemical pump cell.

The concentration cell of the sensor element has a measuring electrodearranged in a measuring-gas region and a reference electrode located ina reference-gas region. The two electrodes are applied on a solidelectrolyte member and electrically connected via the solid electrolytemember. The measuring-gas region in which the measuring electrode islocated is connected to the exhaust gas outside of the sensor elementvia a diffusion barrier and a gas-access hole. The reference-gas regionis in contact with a reference atmosphere via an opening situated on theside of the sensor element facing away from the measuring-gas region.Measuring-gas region and reference-gas region are arranged in the sameplane of stratification of the sensor element configured as layer systemand are separated by a gas-tight separation member. At different oxygenpartial pressures in the measuring-gas region and the reference-gasregion, a so-called Nernst voltage is generated between the measuringelectrode and the reference electrode. When the oxygen partial pressurein the reference-gas chamber is constant, the oxygen partial pressure inthe measuring-gas region may be determined from the Nernst voltage.

The pump cell of the sensor element includes an annular outer pumpelectrode arranged on an outer surface of the sensor element and exposedto the exhaust gas and an also annular inner pump electrode located inthe measuring-gas region on the solid electrolyte member. The inner pumpelectrode may coincide with the measuring electrode of the Nernst cellor may be electrically connected to it. The outer pump electrode has agreater outer radius and a smaller inner radius than the inner pumpelectrode, so that the surface of the outer pump electrode is largerthan the surface of the inner pump electrode. By means of supply lines,the electrodes are electrically connected to contact surfaces arrangedon the side of the sensor element facing away from the electrodes. Aninsulation layer electrically insulates the supply lines of theelectrodes, in particular the supply line of the outer pump electrode,from the solid electrolyte member.

When a pump voltage is applied between the outer pump electrode and theinner pump electrode, the pump cell pumps oxygen ions via the solidelectrolyte member out of the measuring-gas region into the exhaust gasor vice versa from the exhaust gas into the measuring-gas region. Thepump voltage is regulated by an external circuit elements in such a waythat a Nernst voltage of approximately 450 mV is available between theelectrodes of the Nernst cell, which corresponds to an oxygen partialpressure in the measuring-gas region of lambda=1 (stoichiometricair-fuel ratio). Accordingly, oxygen is pumped out of the measuring-gasregion if lean exhaust gas is present (lambda>1), the pump currentflowing in the pump cell being limited by the diffusion stream of theoxygen molecules flowing through the diffusion barrier into themeasuring-gas region. In the case of rich exhaust gas (lambda<1), oxygenis pumped into the measuring-gas region, and the pump current flowing inthe pump cell is limited by the diffusion stream of the gas moleculesthat flow through the diffusion barrier and consume oxygen in themeasuring-gas region (the oxygen pumped into the measuring-gas regionreacts there with the oxygen-consuming gas molecules). In lean exhaustgas, the diffusion stream is proportional to the oxygen concentration ofthe exhaust gas, and in the case of rich exhaust gas it is proportionalto the concentration of oxygen-consuming gas molecules. Thus, it ispossible to ascertain from the pump current the oxygen partial pressureof the exhaust gas, or the partial pressure of the gas moleculesconsuming oxygen.

From German Published Patent Application No. 199 60 329, a gas sensorhaving a similar sensor element is known. In contrast to the sensorelement described in German Published Patent Application No. 199 41 051,the measuring-gas region and the reference-gas region are arranged indifferent planes of stratification. The surfaces of the outer pumpelectrode and the inner pump electrode are identical.

It is disadvantageous in such sensor elements that an overswinger or acounterswing is generated in the sensor signal in response to a changein the direction of the pump current, which occurs during operation ofthe gas sensor in a change from lean to rich exhaust gas, for example.This so-called λ=1 ripple has a detrimental effect on the evaluation ofthe sensor signal.

SUMMARY OF THE INVENTION

The gas sensor according to the independent claim has the advantage thatthe λ=1 ripple is considerably reduced or avoided entirely.

The sensor element includes an electrochemical cell, which has a firstelectrode (outer pump electrode) arranged on an outer surface, facingthe gas, of the sensor element, and a second electrode (inner pumpelectrode, measuring electrode) arranged in a measuring-gas region, aswell as a solid electrolyte member, which is situated between the twoelectrodes and electrically connects them to one another. The firstelectrode is directly exposed to the exhaust gas whose oxygen partialpressure is subject to strong fluctuations. In lean, that is to say,oxygen-rich exhaust gas, the solid electrolyte in the region of thefirst electrode has a high oxygen concentration as well. Since theoxygen in the solid electrolyte is present in the form of ions, a largecharge quantity is formed in the region of the first electrode in thecase of lean exhaust gas. Correspondingly, a small charge quantity ispresent in the region of the first electrode when the exhaust gas isrich and low in oxygen. In contrast, the second electrode is exposed toa largely constant oxygen partial pressure since an oxygen partialpressure of λ=1 is set in the measuring-gas region.

It has been shown that the charge quantity generated in the region ofthe first electrode in lean exhaust gas causes the λ=1 ripple when thepump voltage is reversed. Therefore, to lower the charge quantity, thearea of the first electrode is reduced. Since the charge quantity at thesecond electrode is subject to fewer fluctuations, the area of thesecond electrode may be larger than the area of the first electrodewithout this increasing the λ=1 ripple.

In an advantageous manner, the first and second electrode are designedsuch that, apart from a reduction of the λ=1 ripple, a sufficiently lowresistance between the first and second electrode is achieved as well.When the resistance is low, a relatively low pump voltage is enough togenerate a pump voltage that is sufficient for the control to λ=1. Sincea larger electrode surface means lower resistance, the second electrodetherefore has considerably larger dimensions than the first electrode.If the surface of the first electrode is 0.06 times to 0.6 times as thearea of the second electrode, the λ=1 ripple is reduced in an especiallyeffective manner if the resistance between the first and secondelectrode is sufficiently low. The annular first electrodeadvantageously has an outer radius in the range of 1.1 to 1.7 mm,preferably 1.4 mm, and an inner radius of 0.3 to 0.9 mm, preferably 0.6mm. The outer radius of the annular second electrode is within the rangeof 1.7 to 2.1 mm, in particular 1.9 mm, and the inner radius is within arange of 0.8 to 1.2 mm, preferably 1.0 mm.

In a modification of the present invention, the first and the secondelectrode have an elliptical form and include an elliptical recess, theratio of main axis to auxiliary axis lying within the range of 2:1 to1.1:1, preferably 1.5:1. In sensor elements provided with a heater, atemperature distribution develops in which elliptically shaped areashaving the same temperature are formed in the large areas of the sensorelement, such as on the outer surface on which the first electrode isapplied. Therefore, an elliptical shaping of the electrodes achieves areduction in the temperature differences in different regions of theelectrode surface.

In an advantageous manner, the first and the second electrode include arecess in which a gas-access opening is located via which the gas gainsaccess to the measuring-gas region. Furthermore, the sensor element hasa reference-gas region, which contains a reference air having asufficiently constant oxygen partial pressure. Located in thereference-gas region is a third electrode. The reference-gas region isadvantageously provided in the plane of stratification of themeasuring-gas region.

In the invention described here, the electrode is to be understood asthat region of a printed circuit trace applied on a solid electrolytemember, which is in direct contact with the solid electrolyte member andis thus electrically connected to the solid electrolyte member. Incontrast, the area of the printed circuit trace that is electricallyinsulated from the solid electrolyte member is referred to as supplyline of the electrode. Consequently, the printed circuit trace is calledan electrode in those regions where it is directly applied onto thesolid electrolyte member and renders a contribution to the measuringsignal due to its electrochemical properties. In those regions in whichit is electrically insulated from the solid electrolyte member and doesnot contribute to the measuring signal, or only to a negligible degree,it is called a supply line to the electrode.

In a further development of the present invention, the shortest distancebetween the first electrode and a third electrode arranged in thereference-gas region is markedly greater than the distance between thefirst and the second electrode, this distance corresponding to the layerthickness of the first solid electrolyte member. An increase in thedistance also causes a rise in the resistance between the first andthird electrode, thereby further reducing the in-coupling of the firstelectrode to the third electrode and thus the λ=1 ripple. For thispurpose, the supply line of the first electrode, for example, is atleast regionally arranged in the section that is formed by theperpendicular projection of the second electrode onto the large surfaceof the first electrode. That means that the printed circuit trace of thefirst electrode has a partial region that is located in the area ofprojection of the second electrode onto the large area of the firstelectrode and in which an insulation electrically insulates the printedcircuit trace of the first electrode from the first solid electrolytemember. In a sensor element in which the measuring-gas region and thereference-gas region are located in the same plane of stratification,the insulated partial region is advantageously provided on the side, orabutting against the side, of the first electrode facing thereference-gas region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section along the longitudinal axis of a sensor elementof a first exemplary embodiment of a gas sensor according to the presentinvention.

FIG. 2 shows a section perpendicular to the longitudinal axis of thesensor element of a second exemplary embodiment of the gas sensoraccording to the present invention.

FIG. 3 shows a first plan view of the sensor element of the first andsecond exemplary embodiments of the present invention.

FIG. 4 shows a second plan view of the sensor element of the first andsecond exemplary embodiments of the present invention.

FIG. 5 shows a section along the longitudinal axis of the sensor elementof a third exemplary embodiment of the gas sensor according to thepresent invention, along line V-V in FIG. 6.

FIG. 6 shows a plan view of the sensor element of the third exemplaryembodiment of the present invention.

DETAILED DESCRIPTION

As first exemplary embodiment of the present invention, FIG. 1 shows asensor element 10 of a gas sensor referred to as broadband lambdasensor. Sensor element 10 is configured as a layer system and includes afirst, a second and a third solid electrolyte member 21, 22, 23. Agas-access opening 36 is introduced in first solid electrolyte member21. Arranged between the first and the second solid electrolyte memberis a measuring-gas region 31, a reference-gas region 32, a separationmember 33, a diffusion barrier 34 and a sealing frame 35. Disposed inthe center of flat, hollow-cylindrical measuring-gas region 31 is thelikewise hollow-cylindrical diffusion barrier 34 in whose centergas-access opening 36 discharges. The measuring gas may reachmeasuring-gas region 31 through gas-access opening 36 via diffusionbarrier 34. Separation member 33 forms a gas-tight barrier betweenmeasuring-gas region 31 and reference-gas region 32. Channel-shapedreference-gas region 32 contains a porous material and is in connectionwith a reference atmosphere on the side of sensor element 10 facing awayfrom the measuring region. Measuring-gas region 31 and reference-gasregion 32 are surrounded by a sealing frame 35 on the side.

Arranged on an outer surface of first solid electrolyte member 21 is afirst electrode 41 (outer pump electrode), which is covered by a porousprotective layer 45. A second electrode 42 (measuring electrode, innerpump electrode) is provided in measuring-gas region 31 on the largesurface of first solid electrolyte member 21, this surface lying acrossfrom the outer surface. A third electrode 43 (reference electrode) isprovided in reference-gas region 32 in the plane of stratification ofsecond electrode 42. First electrode 41, together with second electrode42, forms a pump cell, which pumps oxygen into or out of measuring-gasregion 31 with the aid of a external circuit elements. The pump voltagepresent at the pump cell due to the external circuit elements isregulated such that a predefined oxygen partial pressure is present inmeasuring-gas region 31. An oxygen partial pressure of λ=1 is preferablyadjusted, which means that the oxygen partial pressure in measuring-gasregion 31 corresponds to the stoichiometric air/fuel ratio.

The oxygen partial pressure present in measuring-gas region 31 isdetermined by a Nernst cell, which is formed by second electrode 42 andthird electrode 43. The Nernst voltage, caused by different oxygenpartial pressures in measuring-gas region 31 and in reference-gas region32, which—as described earlier—is used to regulate the pump voltage, ismeasured with the aid of the Nernst cell. In an alternative specificembodiment, which is not shown, the electrode associated with the Nernstcell in measuring-gas region 31 and/or the electrode associated with theNernst cell in reference-gas region 32 may be applied on second solidelectrolyte member 22. Furthermore, in addition to second and thirdelectrodes 42, 43 arranged on first solid electrolyte member 21 inmeasuring-gas region 31 and/or in reference-gas region 32, at least oneadditional electrode associated with the Nernst cell may be arranged onsecond solid electrolyte member 22.

A heater 37, which is electrically insulated from surrounding solidelectrolyte members 22, 23 by a heater insulation 38, is providedbetween second solid electrolyte member 22 and third solid electrolytemember 23.

FIG. 2 shows a second exemplary embodiment of the present invention,which differs from the first exemplary embodiment in that themeasuring-gas region and the reference-gas region are not arranged inthe same plane of stratification, but in different planes ofstratification of sensor element 110. Sensor element 110 has a first,second, third and fourth solid electrolyte member 121, 122, 123, 124,respectively. Arranged between first and second solid electrolyte member121, 122 are a measuring-gas region 131, a diffusion barrier 134 and asealing frame 135. The exhaust gas reaches measuring-gas region 131 viaa gas-access opening 136 introduced in first solid electrolyte member121 and via diffusion barrier 134. A reference-gas region 132 isintroduced into third solid electrolyte member 123. A heater 137, whichis embedded in heater insulation 138, is arranged between third andfourth solid electrolyte member 123, 124.

Applied on the outer surface of first solid electrolyte member 121 is afirst electrode 141, which is covered by a porous protective layer 145.In measuring-gas region 131, a second electrode 142 is arranged on firstsolid electrolyte member 121, and a third electrode 143 on the secondsolid electrolyte member. In reference-gas region 132, a fourthelectrode 144 is provided on second solid electrolyte layer 122. Firstand second electrodes 141, 142 form a pump cell together with firstsolid electrolyte member 121; third and fourth electrodes 143, 144 forma Nernst cell together with second solid electrolyte member 122. Thefunctioning method of these electrochemical cells corresponds to that ofthe first exemplary embodiment.

FIG. 3 shows the arrangement of first electrode 41, 141 and secondelectrode 42, 142 on first solid electrolyte member 21, 121 in a firstembodiment of the first and second exemplary embodiments. Porousprotective layer 45, 145 has been omitted to simplify the graphicalrepresentation. First electrode 41, 141 is arranged around gas-accessopening 36, 136 in an annular manner. The inner radius of firstelectrode 41, 141 is 0.6 mm, the outer radius is 1.4 mm. Adjacent tofirst electrode 41, 141 is a supply line 41 a, 141 a, which leads to acontact surface (not shown) on the side of sensor element 10, 110 facingaway from the electrodes. Via the contact surface, first electrode 41,141 is connected to an evaluation circuit arranged outside of the gassensor. Supply line 41 a, 141 a to first electrode 41, 141 iselectrically insulated from first solid electrolyte member 21, 121 by aninsulation layer 47, 147. Insulation layer 47, 147 follows the circularouter contour of first electrode 41, 141 in the transition area betweenfirst electrode 41, 141 and supply line 41 a, 141 a to first electrode41, 141.

Second electrode 42, 142 (shown as dashed line in FIG. 3) is likewisearranged in an annular manner around gas-access opening 36, 136. Itsinner diameter is 10 mm, its outer diameter 20 mm. Thus, the area offirst electrode 41, 141 amounts to approximately half the area of secondelectrode 42, 142. Like the first electrode, second electrode 42, 142and also the other electrodes are electrically contacted by a supplylead (not shown).

FIG. 4 shows a second specific embodiment of the first and secondexemplary embodiments. To simplify the graphical representation, porousprotective layer 45, 145 as well as insulation layer 47, 147 have beenomitted. In the second specific embodiment, first electrode 41, 141 hasan elliptical shape and includes an elliptical recess in whichgas-access opening 36, 136 is arranged. The ratio of main axis toauxiliary axis both of the outer and the inner boundary of firstelectrode 41, 141 is 1.5:1. Like first electrode 41, 141, the secondelectrode (not shown) has an elliptical shape, the area of the secondelectrode being twice as large as the area of first electrode 41, 141.The main axes of the two ellipses of the inner and outer boundary offirst electrode 41, 141 are in parallel to the longitudinal axis ofsensor element 10, 110.

FIG. 5 and FIG. 6 show a third exemplary embodiment of the presentinvention, which differ from the first exemplary embodiment in thedesign of first electrode 241, supply line 241 a to first electrode 241,insulation layer 247 and porous protective layer 245. The other elementsof the sensor element of the third exemplary embodiment have beenprovided with reference signs that match those of the first exemplaryembodiment shown in FIG. 1.

In the third exemplary embodiment, the first printed circuit trace (thatis, first electrode 241 and supply line 241 a to first electrode 241)and the second printed circuit trace (that is, second electrode 42 andthe supply line (not shown) to second electrode 42) have the same form,at least in the area of measuring-gas region 31 of sensor element 10.Thus, the projection of the annularly formed section of the secondprinted circuit trace, that is to say, essentially of electrode 42, ontothe outer surface of first solid electrolyte member 21 correspondsprecisely to the form of the first printed circuit trace in this region.Supply line 241 a of first electrode 241 is electrically insulated fromfirst solid electrolyte member 21 by insulation layer 247. Insulationlayer 247 also extends into an insulated partial region 250 of theprojection of second electrode 242 onto the outer surface of first solidelectrolyte member 21. Insulated partial region 250 abuts against theside of first electrode 241 facing reference-gas region 32 and thirdelectrode 43. Insulation layer 247 essentially consists of aluminumoxide.

Specific embodiments of the third exemplary embodiment are conceivablein which the first printed circuit trace and the second printed circuittrace do not have identical forms in the measuring region of sensorelement 10 either. In particular, first electrode 241 may be smallerthan second electrode 242, that is to say, it may have a smaller outerradius or a smaller inner and outer radius, or have a larger innerradius than second electrode 242.

The present invention is not restricted to the exemplary embodimentsdescribed, but may also be transferred to sensor elements having adifferent configuration in which malfunctions occur as a result of ahigh charge quantity in the region of an electrode applied in the regionof an outer surface of the sensor element.

1. A gas sensor, comprising: a first electrode; a second electrode; anda sensor element including a first solid electrolyte member on which thefirst electrode and the second electrode are arranged, the firstelectrode and the second electrode being electrically connected via thefirst solid electrolyte member, and the first electrode being in contactwith a gas, wherein: an area of the first electrode is smaller than anarea of the second electrode; and in a region of a perpendicularprojection of the second electrode onto a plane of stratification of thefirst electrode, an insulated partial region is provided in which aprinted circuit trace having the first electrode and a supply line tothe first electrode is electrically insulated from the first solidelectrolyte member by an insulation layer.
 2. The gas sensor as recitedin claim 1, wherein: the gas includes an exhaust gas of an internalcombustion engine, and the gas sensor is for detecting at least onephysical magnitude of the exhaust gas.
 3. The gas sensor as recited inclaim 1, wherein: the area of the first electrode amounts to maximally60 percent of the area of the second electrode.
 4. The gas sensor asrecited in claim 1, wherein: the area of the first electrode amounts tobetween 5 and 30 percent of the area of the second electrode.
 5. The gassensor as recited in claim 1, wherein: the first electrode is arrangedon a surface of the sensor element facing the gas, the second electrodeis arranged in a measuring-gas region introduced in the sensor element,and the first solid electrolyte member includes a gas-access opening viawhich the gas is able to enter the measuring-gas region.
 6. The gassensor as recited in claim 5, wherein: at least one of the firstelectrode and the second electrode includes a recess in which thegas-access opening is arranged.
 7. The gas sensor as recited in claim 5,wherein: the first electrode extends to an edge of the gas-accessopening.
 8. The gas sensor as recited in claim 1, wherein: the firstelectrode is at least regionally annular, the at least regionallyannular first electrode includes an outer radius in a range of 1.0 to1.7 mm, and an inner radius in the range of 0.3 to 1.3 mm, the secondelectrode is annular, and the annular second electrode has an outerradius in the range of 1.7 to 2.1 mm, and an inner radius in the rangeof 0.8 to 1.2 mm.
 9. The gas sensor as recited in claim 1, wherein: thefirst electrode is at least regionally annular, the at least regionallyannular first electrode includes an outer radius of 1.2 mm, and an innerradius of 1.0 mm, the second electrode is annular, and the annularsecond electrode has an outer radius of 1.9 mm, and an inner radius of1.0 mm.
 10. The gas sensor as recited in claim 1, wherein: at least oneof the first electrode and the second electrode has an elliptical shapewith an elliptical recess, a ratio of a main axis to an auxiliary axisof the elliptical shape 1.5:1, and the main axis is parallel to alongitudinal axis of the sensor element.
 11. The gas sensor as recitedin claim 1, wherein: at least one of the first electrode and the secondelectrode has an elliptical shape with an elliptical recess, and a ratioof a main axis to an auxiliary axis of the elliptical shape is in therange of 2:1 to 1.1:1, the main axis being parallel to a longitudinalaxis of the sensor element.
 12. The gas sensor as recited in claim 1,wherein: at least one of a measuring-gas region, a reference-gas region,the second electrode arranged in the measuring-gas region, and a furtherelectrode arranged in the reference-gas region are situated in the sameplane of stratification of the sensor element.
 13. The gas sensor asrecited in claim 1, further comprising: an additional electrode,wherein: the sensor element includes an electrochemical cellcorresponding to a Nernst cell, the Nernst cell includes in a measuringgas region the second electrode, and the second electrode and theadditional electrode are electrically connected by the first solidelectrolyte member.
 14. The gas sensor as recited in claim 1, wherein:the second electrode is arranged in a measuring-gas region introduced inthe sensor element, a reference-gas region is provided in a plane ofstratification of the measuring-gas region, and the insulated partialregion is provided so as to abut a side of the first electrode facingthe reference-gas region.
 15. The gas sensor as recited in claim 1,further comprising: a third electrode arranged in a reference-gasregion, wherein: a shortest distance between the first electrode and thethird electrode is larger by at least 50% than a layer thickness of thefirst solid electrolyte member.